I run the system at 190FSB, but cannot go any higher without encountering stability issues, even though the CPU temp doesn't go above 46c when at 200FSB.

I am considering selling existing CPU on ebay, and buying an Athlon XP 2500-M, so I can pump up the FSB and get the maximum performance from RAM and motherboard. What sort of overclock could I expect from the Corsair Ram the motherboard and the XP 2500-M, and does anything think it would be worth it? It will be my last upgrade before going with an Athlon 64 probably in 18 months time.

People have been got 2.4ghz roughly from xp2500-m's (i have seen a few 2.6/7ghz) www.cpucitystore.co.uk is normally a good place to look.

The RAM should get you to 200fsb, what timings and volts are you running?
Have you tried upping the voltage a bit on the CPU? And if you can post the code on the chip people should be able to tell you if it is a good overclocker.

It would only cost about £20 altogether to upgrade, as you can get £40ish for your chip, and then £60/70 for a xp2500-m (buy it second hand and obviosuly it will be cheaper, and you can ask what they have clocked it to.)

My Ram is running in sync with the CPU at 190 FSB. Timings are 8 3 3 2, using stock voltages.

The voltage for the CPU is 1.775 and at 190FSB it is rock solid. I can run the CPU at 200FSB with voltage at 1.85 (max on my board), and I can run Farcry, FS2004 ok, but when I do MP3 encoding, video encoding and watch videos the PC freezes and have to reboot. I have tried decreasing the speed of the memory and the same happens, so the memory seems to be fine. This has led me to believe that the CPU just isn't a great overclocker. Unfortunately I don't have the code on the CPU, don't really want to take heatsink off at the moment.

That does sound like the CPU is at its limits. You should be able to tighten the timings on the RAM to 2-2-2-11 at that speed (NForce 2 runs best aith a tRAS of 11 for some reason). As blockers says a mobile Barton should see you to 2.3-2.4GHz @ 200FSB without any major difficulty. Don't worry about the stepping, it makes no real difference.